It has been known in a large number of optical devices to form a groove for supporting an optical fiber, the groove having a V-shaped or inverted-trapezoidal-shaped cross section, by forming a predetermined mask film on a single crystal silicon substrate (a wafer) and then anisotropic etching the substrate (for example, see Patent Publications 1-3 listed later). The V-shape or inverted trapezoidal shape of the cross section is caused by differences in etching rates due to different crystal orientations. Generally, the anisotropic etching is performed by forming the mask film made from SiO2 on a silicon single crystal substrate, and then by wet etching with an etching solution such as a potassium hydroxide, etc.
FIG. 5 is a cross-sectional view of a silicon substrate showing progress of an etching process when a groove having a V-shaped cross section or an inverted trapezoidal cross section is formed, and FIG. 6 is a graph showing the relationship between a wet-etching time and a depth of the groove. As shown in FIG. 5, when a mask film 52 on a silicon substrate 50 is formed with an aperture 52a having a width WM, an etching process progresses in the order (a), (b), (c) in FIG. 5. During the progress of the etching process, a change in a width WV of the groove 54 (i.e., a distance between upper edges of inclined surfaces 54a) is small, while a depth DV of the groove 54 becomes large. FIG. 5(a) shows a state SA in which the inclined surfaces 54a of the groove 54 are formed to a location PA at which the optical fiber F contacts the groove 54 when the former is supported on the latter. FIG. 5(b) shows a state SB in which the inclined surfaces 54a of the groove 54 are formed to a location PB and thus the optical fiber F can be supported on the groove 54. FIG. 5(c) shows a state SC in which the inclined surfaces 54a of the groove 54 are formed until a shape of a cross section of the groove 54 becomes a V-shape. Further, as shown in FIG. 6, the depth of the groove 54 is in proportion to the wet-etching time. The times TA, TB, TC shown in FIG. 6 are respective times corresponding to the states SA, SB, SC.
FIG. 7 is a view showing the relationship between the width WM of the aperture and the width of the groove (i.e., the distance between the upper edges of the inclined surfaces) WV when there is an angle between a direction in which an edge of the aperture 52a of the mask film 52 extends and a predetermined crystal orientation of the substrate 2 (hereinafter, said angle is referred to as “an orientation misalignment”). When there is such an orientation misalignment, as shown in FIGS. 7(a) and 7(b), the width WV of the groove is larger than the width WM of the aperture. FIG. 7(a) shows a state in which the orientation misalignment is relatively small, while FIG. 7(b) shows a state in which the orientation misalignment is relatively large. As can be seen from FIGS. 7(a) and 7(b), the larger the orientation misalignment is, the larger a side-etched amount is, the side-etched amount being a difference δ between the width WV of the groove and the width WM of the aperture.
FIG. 8 is a view showing the relationship between the etching time and the variation in the width of the groove when there is an angle or an orientation misalignment between a direction in which the edge of the aperture 52a of the mask film 52 extends and the predetermined crystal orientation of the substrate 2. The times TA, TB, TC shown in FIG. 8 are respective times corresponding to the states SA, SB, SC shown in FIG. 5. As can be seen from FIG. 8, when the etching time becomes longer than a certain time TD, the variation in the width WV of the groove gradually becomes large. Further, the larger the orientation misalignment is, the larger the variation in the width WV of the groove is. Preferably, a standard deviation of the variation in the width WV of the groove is equal to or less than 0.18 μm.
Therefore, in order to reduce such variations in the width WV of the groove, it is preferable that the etching time be set so as to be less than the time TD shown in FIG. 8. However, when the etching time is set so as to be less than the time TD, a problem that the optical fiber F and the substrate 2 interfere with each other is caused as shown in FIG. 5(a),
Regarding this problem, the Patent Publication 4 listed later presents two solutions. FIG. 9 is a cross-sectional view of a prior art groove showing a first solution, and FIG. 10 is a cross-sectional view of a prior art groove showing a second solution.
As shown in FIG. 9, in the first solution, an etching process for forming the inclined surfaces 54a is finished between the state SA and the state SB shown in FIG. 5, a piece of the optical device is subsequently cut away from the wafer, and the through-bore 56 is individually made by means of a dicing process.
As shown in FIG. 10, in the second solution, an etching process for forming the inclined surfaces 54a is finished between the state SA and the state SB shown in FIG. 5, a wafer is inverted upside down, and the wafer is etched from a surface 50b opposite to the surface 50a on which inclined surfaces 54a are formed in order to form a through-bore 58 for removing a portion of the substrate 50 which portion interferes with the optical fiber F.    Patent Publication 1: Japanese Patent Laid-open Publication No. 10-199856    Patent Publication 2: Japanese Patent Publication No. 2771167    Patent Publication 3: Japanese Patent Publication No. 2949282    Patent Publication 4: Japanese Patent Publication No. 3014035